1. A Device which Enhanced the Utility of Fire!!! A True Mediator !!!
HEAT EXCHANGERS By
P M V Subbarao
Associate Professor
Mechanical Engineering Department
I I T Delhi
A Device which Enhanced the Utility of Fire!!!
A True Mediator !!!

2. Fire Can only Heat Solids !!!!!

3. EARLIEST TYPES OF HX : COOKING
Primitive humans may first have savoured roast meat by chance, when the flesh of a beast killed in a forest fire was found to be more palatable and easier to chew and digest than the customary raw meat.
They probably did not deliberately cook food, though, until long after they had learned to use fire for light and warmth.
It has been speculated that Peking man roasted meats, but no clear evidence supports the theory.
During Palaeolithic Period, Aurignacian people of southern France apparantly began to steam their food over hot embers by wrapping it in wet leaves.
Crude procedures
as toasting wild grains on flat rocks and using shells, skulls,
or hollowed stones to heat liquids.
Introduction of pottery during the Neolithic Period.
A paste, toasted to crustiness when dropped on a hot stone, made the first bread.

4. Cauldron : A Formal Heat Exchanger
In 130BC. Hero, a Greek mathematician and scientist is credited with inventing the first practical application of steam power, the aelopile.
Simply a cauldron with a lid, the aelopile had two pipes that channeled steam into a hollow sphere.
The sphere, which pivoted on the steam pipes, had two nozzles situated on opposite sides of its axis.
Thus, the cauldron was fired, the water in it boiled, the steam was channeled into the sphere, and as the steam escaped through the nozzles, the sphere would spin.
It was a thought device and a novelty.

5. Mediation is Necessary When Things Grow Big !!!!

6. Milk Pasteurizers
The milk is piped into a pasteurizer to kill any bacteria.
The most common is called the high-temperature, short-time (HTST) process in which the milk is heated as it flows through the pasteurizer continuously.
Whole milk, skim milk, and standardized milk must be heated to 72° C for 15 seconds.
The hot milk passes through a long pipe whose length and diameter are sized so that it takes the liquid exactly 15 seconds to pass from one end to the other.
A temperature sensor at the end of the pipe diverts the milk back to the inlet for reprocessing if the temperature has fallen below the required standard.

7. Milk Pasteurizers

8. Crude Oil Distillation

9. Basic Method of Heat Communication
Hot Fluid
Thermal Structure HX
Cold Fluid
Convection HT
Drop in Enthalpy
of Hot Fluid
Rise in Enthalpy of
Cold Fluid
Mechanism of Heat Transfer
Donor
Thermal Structure
Acceptor

10. Thermodynamic Perspective of HX.
The energy absorbed by cold fluid
The convective heat lost by hot fluid
Thermal Energy Balance:

11. Heat Transfer Perspective of HX.
How can A hot fluid loose thermal energy?
How much A hot fluid can loose thermal energy?
What is the mutual interaction?
Generalized Newton’s Law of Cooling.
Overall Coefficient of Heat Transfer, U

12. Creative Ideas for Techno-economic Feasibility of HX.
For a viable size of a HX
How to maximize Effective area of heat communication?.
How to maximize Overall Heat transfer coefficient?
Should we decrease or increase Effective temperature difference?

13. Recuperation Vs Regeneration

14. Direct Vs Indirect Contact
Direct Contact Hx
Indirect Contact Hx

15. Single Phase Vs Multi Phase HX
Multi Phase Boiling HX
Single Phase HX
Multi Phase Condensing HX

16. Geometry
Tubular Hx
Planar Hx
Extended Surface Hx

17. Study of Simple Heat Exchangers
Parallel Flow Heat Exchanger:
Counter Flow Heat Exchanger:

18. Theory of Simple Heat Exchangers
Infinitesimal adiabatic Heat Exchanger model.
For Infinitesimal Heat communication between cold and hot.

19. Energy Balance in infinitesimal Heat Exchanger
Energy balance in an Infinitesimal adiabatic Heat Exchanger model.
Energy Balance:

20. Thermal Resistance of infinitesimal Heat Exchanger

21. Heat Transfer between two fluids separated by finite surface wall

22. Basic Counter Flow HX :A Perfect Heat Transfer Device
Local temperature difference for heat communication:
Basic Counter Flow HX :A Perfect Heat Transfer Device

23. Synergism between HT & TD:

24. For A finite HX:

25. For A finite HX:

26. Newton’s Law for Combined Cooling & Heating
A representative temperature difference for heat communication:
Also called as Overall temperature driving force.

27. Discussion on LMTD
LMTD can be easily calculated, when the fluid inlet temperatures are known and the outlet temperatures are specified.
Lower the value of LMTD, higher the value of overall value of UA.
For given end conditions, counter flow gives higher value of LMTD when compared to co flow.
Counter flow generates more temperature driving force with same entropy generation.
This method is a capacity based design.

28. A Model Problem
A concentric tube heat exchanger is used cool the lubricating oil for a large power plant gas turbine. The flow rate of cooling water through the inner tube(di = 25mm) is 0.2 kg/sec, while the flow rate of oil through the outer annulus (do= 45 mm) is 0.1 kg/sec.
The oil and water enter at temperatures of 100 and 30 0C, respectively.
Calculate the value of LMTD for both Co and counter flow Hxs, if outlet temperature of the oil is 600C.

29. Parameters of Fluids Inlet temperature of hot oil = 1000C
Outlet temperature of hot oil = 600C
Mass flow rate of oil = 0.1 kg/sec.
Specific heat of oil = kJ/kg.
Inlet temperature of cooling water = 300C
Mass flow rate of cooling water = 0.2 kg/sec.
Specific heat of cooling water = kJ/kg.

30. Conservation of Energy
Rate of Heat lost by hot oil = Rate of heat gained by cooling water

31. Counter flow Model of Hx
Hot Oil
Cooling Water
LMTD = C

32. Co flow Model of Hx
Hot Oil
Cooling Water
LMTD = 39 0C

33. Comparison of Co-flow and Counter flow
Overall heat transfer coefficient, U = 37.7 W/m2K.
LMTD for Co-flow = 39 0C.
LMTD for Counter-flow = 43.20C.
Length of Hx for co-flow = 73.7 m
Length of Hx for Counter flow = 66.5 m